Process plants making pH measurements want accurate, reliable performance with a reasonable electrode life expectancy while minimizing maintenance. A quality pH sensor system, when undamaged, cleaned and properly calibrated, will provide such performance.
However, an electrode — even in a process that doesn’t cause coating, plugging, abrasion or any other problems — still requires periodic calibration to correct for sensor aging and non-recoverable changes to the electrode. Because these aging effects happen slowly, you shouldn’t have to calibrate more often than once a month in typical general-purpose applications. The need for more frequent calibration stems from a specific cause, i.e.:
• an aggressive process;
• ineffective electrode cleaning;
• improper routine calibration;
• overly temperature dependent pH; or
• incorrect electrode selection.
Dirty or faulty electrodes can cause anything from slow response to a completely erroneous reading. For example, if a film remains on the pH sensor after cleaning, you might misinterpret the resulting measurement error as a need for re-calibration. Correct cleaning can reverse these changes and, so, is a key maintenance step.
The accuracy of pH measurements depends upon maintenance; maintenance frequency largely depends upon the application. Understanding and addressing the causes of pH measurement difficulties are key to ensuring stable and accurate readings. However, troubleshooting a pH system can pose challenges. The guidelines presented here are a good starting point for tackling problems.
Insights From Calibration
Troubleshooting has four main parameters: 1) asymmetry/zero; 2) slope; 3) measuring electrode impedance; and 4) reference electrode impedance.
Most commercial instruments give asymmetry/zero and slope readings. A pH system with a solution ground/liquid ground and advanced sensor diagnostics also will provide impedance values. Understanding the purpose of each of these values allows you to know where problems lurk — and where to start your search for answers.
The asymmetry potential (AS), also referred to as the millivolt offset, indicates the condition of the reference electrode. Theoretically when the electrodes are placed in a pH-7 buffer solution, the millivolt output from the electrode pair (pH and reference) should be zero. An asymmetry reading of 20 means the pH sensor is generating 20 mV instead of the expected 0 mV.
The reference sensor causes most asymmetry problems. Some millivolt offsets stem from potassium chloride (KCl) depletion from the reference electrolyte or poisoning of the reference electrolyte with process solution. When the offset is ±30 mV or more, you should replace the reference electrode. Per the Nernst equation, a change in pH of 1 results in 59.16 mV at 25°C. If your pH system has 30 mV of offset, you are adjusting for an incorrect reading equal to 0.5 pH.
The slope, also referred to as the efficiency of the electrode, is an indication of the condition of the measuring (glass) electrode. The slope is given as a percentage value, with 100% being ideal. A new electrode should have a slope in the upper-90% range. As the electrode ages and loses efficiency, the slope and response will start to decrease.
The slope value is updated each time you perform a two-point calibration; you should detect only small changes in the value. (Table 1 lists slope reading issues and actions.) Inadequate cleaning can lead to a coating buildup (Figure 1) that causes a low slope value; to avoid this, clean the electrode as needed with a 5–10%-HCl solution for a minute, rinse thoroughly with clean water, soak and recalibrate. Replace the pH electrode when the slope value is in the mid- to low-80% range.
Readings from the reference impedance (RZ), also referred to as the resistance or the reference junction, can indicate the need to clean out a precipitate blockage forming in the reference junction. (Table 2 lists causes and remedial actions for RZ alarms.) The conductivity of the process solution also influences this resistance.
Typically, a clean reference junction will have a resistance of less than 10–15 kΩ but, in low conductivity solutions, RZ values between 200 kΩ and 500 kΩ aren’t uncommon. When the RZ value starts to approach 30–35 kΩ, the electrode will begin to have a slow upward drift. When the reference impedance exceeds 100 kΩ, an error message should appear on the instrument connected to the pH sensor.